Some of the prior art includes chain conveyors such as power and free conveyors where the load supporting carrier uses hooks, tabs, or dogs to catch a pusher, hole or depression on the moving chain. Besides having a chain that is noisy, this particular type of conveyor suffers from a requirement for lubrication of the chain for proper maintenance and durability and, in effect, provides a dirty system. To disengage the load-supporting carriers, one needs to pivotally or slidably disengage the hook or tab from the moving chain. The moving chain also includes numerous hooks, or pushers to decrease the wait time for engagement and for carrier motion.
Other forms of conveyors utilized pallets that are driven by friction from the supporting conveyor rollers where disengagement or stopping the load supporting portion is achieved by extending a cam, raising a fixed stop, or extending a tab on the pallet. This form of conveyor is limited to gravity for generating frictional forces and thus is generally less useful for inclined conditions. Furthermore, speed of the pallets will vary in the case where loaded pallets create more friction and thus move faster than empty pallets.
Still other forms of conveyors have sought to optimize frictional engagement on the load supporting means by using opposing belts or opposing wheels to pinch the load supporting means with forces derived by actuators external to the load supporting means. This prior art is limited in its ability to permit independent travel or stoppage of the load-supporting portion. One example would be a contiguous line of driveless pallets driven by the last pallet in line which is pinched by drive rollers or driveless pallets each pinched by drive rollers. Another example is evidenced by parallel drive belts where all of the pallets in a given conveyor section receive the same drive force that adjacent pallets receive, or each pallet must be acted upon by a separate drive means leading to numerous drive sections.
It is believed that none of the conveyors previously proposed permits independent travel or stoppage of adjacent load supporting portions while employing a single drive means which optimize frictional drive forces while avoiding tabs or dogs for positive engagement with the drive means.